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Many dads will focus much more on taking care of their family and children than of themselves. Part of the reason for that is that many dads are afraid to find out all the things they need to do to be healthier and then all the arguments or excuses they will have to make when they don't follow through. Sometimes it's simply being stubborn and not wanting to change. And by the way, nagging spouses often aren't that successful in getting them to do it.
If however you're one of his kids and instead of lecturing him, you tell him how afraid you are that he'll have a heart attack, stroke, etc. if he continues to not take care of himself, you might break through. When you speak to him, come more from your fear of his dying than from your frustration with him. In fact, the more emotional you get about that fear, the more effective you'll be.
Also try the sales technique called the assumptive close. To do that say to him, "I/we are scheduling (he can always change the time) an appointment with your doctor and/or a nutritionist and/or a physical therapist and/or a personal trainer. What day of the week is best?" Then say, "And we're going with you to it!" (to further avert his wiggling out of it).
Perhaps the most effective way to motivate your dad to take better care of himself is to have "skin in the game" yourself. To do that say to him, "I'll make a trade. If you do a, b and c to take better care of yourself, that you don't want to do but which matter to me, I will do x, y and z to improve or fix things in my life, that you'd like me to do that I don't want to do... and you get to choose what they are."
You may not want to use the latter approach, but it may be the most effective.
As a dad I can think of several things that would make my children's lives better if they did them that I'd trade with them taking better care of myself in a heartbeat (pun intended).
